1. Field of the Invention
This disclosure relates to an electrochromic device with improved display properties and a method for fabricating the same. Other exemplary embodiments include an electrochromic device that comprises an electrochromic layer, a reflective layer and an electrolyte layer, that minimizes cross-talk and image diffusion since the electrolyte layer is present in only an active region of unit pixels.
2. Description of the Related Art
“Electrochromism” is defined as a phenomenon in which the inherent color of some chemical species change reversibly according to the direction of an applied electric field. Based on such a phenomenon, an electrochromic material shows reversible changes in optical properties due to electrochemical oxidation/reduction reactions. More specifically, an electrochromic material exhibits no color when no electric field is externally applied, but exhibits its inherent color when an electric field is applied, or alternatively, an electrochromic material exhibits its color when no electric field is externally applied, but exhibits no color when an electric field is applied. Inorganic compounds (e.g. oxide tungsten or oxide molybdenum) and organic compounds (e.g. pyridine or aminoquinone compounds) display electrochromic properties.
Since electrochromic devices have advantages of high reflectivity in the absence of any external light source, superior flexibility, portability and light-weight, they are expected to be utilized in a variety of applications in flat panel displays. In particular, electrochromic devices have been the focus of intense interest lately owing to their potential applications in electrical papers (E-paper), which are actively researched as an electronic media capable of serving as substitutes for paper.
FIG. 1A is a cross-sectional view schematically illustrating the structure of a comparative electrochromic device. As shown in FIG. 1A, the electrochromic device has a structure in which an electrochromic material 20 is coated on an upper transparent electrode 10, and a counter material 40 and a light-reflective material 30 are coated on a lower counter electrode 50, the respective coated surfaces of the respective electrodes facing (being opposedly disposed to) one another.
The mechanism by which the electrochromic device of FIG. 1A displays white and specific colors will be explained with reference to FIGS. 1B and 1C.
When no electric field is applied, the electrochromic material 20 is transparent, thus transmitting light. On the other hand, when an electric field is applied, the electrochromic material 20 is oxidized or reduced, thus exhibiting its inherent color. That is to say, when no electric field is applied, since the electrochromic material 20 is transparent, it does not absorb any wavelength of light. Accordingly, all wavelengths of incident light passing through the upper transparent substrate 10 are reflected by the lower reflective layer 30 and are thus emitted back through the upper transparent electrode 10 at the front of the device. For this reason, an observer perceives a white color at the front of the display device (FIG. 1B).
On the other hand, when an electric field is applied, since the electrochromic material 20 is either oxidized or reduced, it shows its inherent color and absorbs all except the inherent color of wavelengths. Accordingly, only a wavelength of light, which has the same color as that the electrochromic material 20 passes through the upper transparent electrode 10 and the remaining wavelengths of light are absorbed in the electrochromic material 20. Only the inherent color of light from the oxidized or reduced electrochromic material 20 is emitted from the lower reflective layer through the upper transparent electrode 10 at the front of the device. For this reason, an observer perceives the inherent color at the front of the display device (FIG. 1C).
Similar to general devices, electrochromic devices realize color representation through unit pixel arrays, which represent colors of red, green and blue, respectively. However, conventional electrochromic devices have a disadvantage in that they cannot secure normal display properties because of crosstalk between adjacent pixels, which is an undesirable phenomenon resulting from the ionic conductivity of the electrolyte.
Viologen, which is the most intensely studied one of organic-based electrochromic materials, will be exemplified to explain the cause of the crosstalk.
When an electric field is applied to the space between opposite electrodes of an electrochromic device, viologen reacts with ions or electrons contained in an electrolyte, as depicted in the following reaction scheme 1, thus exhibiting variation in color.

That is, a color change of viologen to the blue color (induced by the application of an electric field) is based on the fact that a radical mono cation gains an electron (i.e. reduced) derived from the electrolyte in oxidation/reduction reactions.
However, in another comparative electrochromic device as shown in FIG. 2, since an electrolyte fills all the space between unit pixels of the upper and lower electrodes, crosstalk or image diffusion inevitably occurs between adjacent unit pixels due to the ionic conductivity of the electrolyte.
As shown in FIG. 3, when only a specific pixel by selecting X and Y lines is intended to be driven, the ionic conductivity of the electrolyte causes crosstalk or image diffusion in which adjacent pixels as well as the specific pixel are driven. As a result, electrochromic devices present disadvantages in that they cannot secure normal display properties.